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correspondence principle, philosophical guideline for the selection of new theories in physical science, requiring that they explain all the phenomena for which a preceding theory was valid. Formulated in 1923 by the Danish physicist Niels Bohr, this principle is a distillation of the thought that had led him in the development of his atomic theory, an early form of quantum mechanics.

Early in the 20th century, atomic physics was in turmoil. The results of experimentation presented a seemingly irrefutable picture of the atom: tiny electrically charged particles called electrons continuously moving in circles around an oppositely charged and extraordinarily dense nucleus. This picture was, however, impossible in terms of the known laws of classical physics, which predicted that such circulating electrons should radiate energy and spiral into the nucleus. Atoms, however, do not gradually lose energy and collapse. Bohr and others who tried to encompass the paradoxes of atomic phenomena in a new physical theory noted that the old physics had met all challenges until physicists began to examine the atom itself. Bohr reasoned that any new theory had to do more than describe atomic phenomena correctly; it must be applicable to conventional phenomena, too, in such a way that it would reproduce the old physics: this is the correspondence principle.

The correspondence principle applies to other theories besides quantum theory. Thus the mathematical formulations for the behaviour of objects moving at exceedingly high speeds, described by relativity physics, reduce for low values of speed to the correct descriptions of the motions of daily experience.

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